Functional Safety KFD2-SR2-Ex*.W(.LB)
Planning
2020-05
11
3.2 Assumptions
The following assumptions have been made during the FMEDA:
• Failure rates are constant, wear is not considered.
• Failure rate based on the Siemens standard SN 29500.
• The safety-related device is considered to be of type A device with a hardware
fault tolerance of 0.
• External power supply failure rates are not included.
• Only one input and one output are part of the safety function (only for 2-channel version).
• Short circuit (SC) detection and lead breakage (LB) detection are enabled.
• The device will be used under average industrial ambient conditions comparable
to the classification "stationary mounted" according to MIL-HDBK-217F.
Alternatively, operating stress conditions typical of an industrial field environment similar
to IEC/EN 60654-1 Class C with an average temperature over a long period of time
of 40 ºC may be assumed. For a higher average temperature of 60 ºC, the failure rates
must be multiplied by a factor of 2.5 based on experience. A similar factor must be used
if frequent temperature fluctuations are expected.
SIL 2 application
• To build a SIL safety loop for the defined SIL, it is assumed as an example that this device
uses 10 % of the available budget for PFD
avg
/PFH.
• For a SIL 2 application operating in low demand mode the total PFD
avg
value
of the SIF (Safety Instrumented Function) should be smaller than 10
-2
,
hence the maximum allowable PFD
avg
value would then be 10
-3
.
• For a SIL 2 application operating in high demand mode the total PFH value
of the SIF should be smaller than 10
-6
per hour, hence the maximum allowable PFH value
would then be 10
-7
per hour.
• Since the safety loop has a hardware fault tolerance of 0 and it is a type A device,
the SFF must be > 60 % according to table 2 of IEC/EN 61508-2 for a SIL 2 (sub) system.
SILCL and PL Application
• The device was qualified for use in safety functions up to SIL2 acc. to IEC/EN 61508.
The risk reduction is equivalent to PL d acc. to EN/ISO 13849-1 or to SILCL2 acc. to
IEC/EN 62061. See chapter 4 of EN/ISO 13849-1 for details on comparison between
SIL and PL statements.
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